1. Field of the Invention
The present invention relates to a limit electric current type oxygen sensor capable of detecting a density of oxygen in an internal combustion engine.
2. Description of the Related Art
The most popular type of oxygen sensor is an oxygen density cell having a body formed of a solid electrolyte material such as zirconia. In this type of oxygen sensor, an electromotive force is generated when the air-fuel ratio of the gas to be detected is higher than a predetermined air-fuel ratio value corresponding to a chemical stoichiometric value, i.e., a theoretical air-fuel ratio, and therefore, this type of sensor can only discriminate whether or not an electromotive force has been generated, i.e., the sensed air-fuel ratio is lower or higher than the theoretical air-fuel ratio.
Accordingly, a limit electric current type oxygen sensor has been proposed whereby a detection of the wide range of values of the air-fuel ratio between the lean side and the rich side of the air-fuel ratio is possible. This limit electric current type oxygen sensor usually comprises a body formed of a solid electrolyte material, a perforated oxygen diffusion speed control layer formed on the solid electrolyte body and an electrode arranged on the solid electrolyte body in such a manner that it is covered by the diffusion speed control layer. In this type of sensor, when an electric voltage is applied to the electrode, an electric current is generated in the solid electrolyte body, and the generated electric current basically varies in accordance with the electric voltage applied when the oxygen density remains at a predetermined value. However, a range of values of the electric voltage applied to the electrode exists at which the value of the electric current is unchanged (or is saturated) due to the provision of a diffusion speed limit area for controlling the speed of the diffusion of oxygen ions via the perforations in the oxygen diffusion speed control layer. This value of the electric current in the saturated zone, called the limit electric current, has a predetermined proportional relationship to the oxygen density, and therefore, in a known device an electric current (i.e., pumping electric current) applied to the sensor is controlled to maintain the voltage applied to the electrodes at a predetermined value, so that the saturating electric current corresponds to the oxygen density. Namely, the value of the oxygen density can be determined from the value of the detected electric current.
In this type of oxygen sensor, wherein the diffusion velocity control layer is formed of a perforated material, the diffusion velocity is greatly dependent on the temperature of the sensor, and therefore, a temperature compensation circuit must be included for compensating the effect of the temperature on the detected value of the oxygen density, which requires a complicated construction of the sensor.
In view of the above, a limit electric current type oxygen sensor was proposed in which a pin-hole is employed as the diffusion speed control means (see Japanese Un-Examined Pat. Publication No. 56-130649). In this pin-hole type sensor, a diffusion chamber is formed on one side of the solid electrolyte body and is connected, via the pin-hole, to an area in which the gas to be detected is located. The pin-hole controls the speed of the diffusion of oxygen ions such that the oxygen density of the gas in the area on one side of the pin-hole is the same as the oxygen density on the other side of the pin-hole, i.e., in the diffusion chamber. The diffusion via the pin-hole is a particle diffusion process, which has a much smaller temperature dependency with regard to the diffusion speed than the diffusion carried out via the conventional perforation under the Knudsen diffusion principle. Therefore, the pin-hole type sensor has an advantage in that a relatively simple circuit can be used to compensate the temperature dependency, to thereby detect a precise value of the air-fuel ratio.
In this pin-hole type diffusion oxygen sensor, the diffusion velocity depends greatly on the pressure parameter. Namely, the precision of the value of the oxygen density as detected is influenced by a pressure pulsation, for example, in the intake passageway of an internal combustion engine, i.e., the oxygen particles are introduced into or discharged from the diffusion chamber, via the pin-hole, in accordance with an increase or decrease generated by the pulsation of the intake pressure. Nevertheless, the speed of introduction of the oxygen particles and the speed of exhaust of the oxygen particles are not the same, i.e., the speed of introduction of the oxygen particles into the chamber due to the increase in the pressure is higher than the speed of the exhausting of the oxygen particles from the chamber. Accordingly, to evenly balance the introduction of the oxygen into the chamber and the exhaust of the oxygen from the chamber, the amount introduced must be larger than the amount exhausted, and therefore, a larger electric current is obtained than would be if a pressure pulsation did not occur, even when the oxygen density is unchanged.